Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits. This property, called superplasticity, is exhibited by certain metals and alloys within limited ranges of temperature and strain rate. For example, titanium and its alloys are superplastic in the temperature range from about 1450–1850° F. (785–1010° C.).
Superplastic forming (SPF) is a technique for expanding or stretching metal that relies on superplasticity. A typical SPF process involves placing one or more sheets of metal in a die, heating the sheets to an elevated temperature within the superplastic range for that metal, and superplastically forming the sheet(s) at the SPF temperature. Expansion can and often does exceed 100%. Generally, a differential forming pressure from a gas manifold is injected between sealed sheets and is used as the driving force to stretch the sheet(s) into the desired shape against the shaped surfaces of supporting dies. SPF can be called “blow molding” insofar as it uses differential gas pressure to form the material. The differential pressure is selected and controlled to strain the material at a strain rate that is within its superplastic range. The following patents are illustrative of SPF processes and equipment:
PATENTTITLEISSUE DATE3,920,175Method of SPF of Metals withNov. 18, 1975Concurrent Diffusion Bonding3,927,817Method for Making MetallicDec. 23, 1975Sandwich Structures3,605,477Precision Forming of TitaniumSep. 29, 1971Alloys and the Like by Use ofInduction Heating4,141,484Method of Making a MetallicFeb. 27, 1979Structure by Combined FlowForming and Bonding4,649,249Induction Heating Platen for HotMar. 10, 1987Metal Working4,117,970Method for Fabrication ofOct. 3, 1978Honeycomb Structures5,024,369Method to ProduceJun. 18, 1991Superplastically FormedTitanium Alloy ComponentsWe incorporate these patents by reference.
One advantage of SPF is the forming of complex shapes from sheet metal while reducing the time and eliminating the waste of milling. SPF sandwich panel production results in a considerable cost saving and reduces total part count over conventional “built up” assemblies that are arranged and fastened together. In addition, the SPF process is generally applicable to single and multisheet fabrication. For multisheet fabrication, SPF is combined with joining processes, such as diffusion bonding, brazing, or laser welding, to produce complex sandwich structures. In the present invention, we join the sheets with adhesive bonding. The SPF process produces lighter, lower cost parts that use fewer fasteners. Use of SPF is accelerating for the manufacture of parts for aircraft, missiles, and spacecraft. In the present invention, we combine SPF with adhesive bonding to make multisheet sandwich panels, especially panels made from aluminum or its SPF alloys.
Titanium superplastically-formed/diffusion-bonded (SPF/DB) panel structures can cost 50% less than conventional honeycomb construction. The SPF/DB process can produce tailored rib or integral hard point and fastener through-hole structures, such as those described in published PCT Application US96/20115, which we also incorporate by reference.
In a typical prior art SPF process for titanium or its alloys, the sheet metal is placed between dies at least one of which has a contoured surface corresponding to the shape of the product. The dies are placed on platens, which are heated, generally using embedded resistive heaters. The platens heat the dies to about 1650° F. (900° C.). Because the titanium will readily oxidize at the elevated temperature, an inert gas, such as argon, surrounds the die and workpiece. The dies heat the sheet metal to the temperature range where the sheet metal is superplastic. Then, under applied differential pressure, the sheet metal deforms against the contoured surface.
The platens and dies have a large thermal mass. They take considerable time and energy to heat and are slow to change their temperature unless driven with high heat input or with active cooling. To save time and energy, the platens must be held near the forming temperature throughout a production run (i.e., the production of a number of parts using the same dies), so loading raw materials and unloading completed parts is a challenge. The raw sheet metal must be inserted onto the dies, and formed parts removed, at or near the elevated forming temperature. The hot parts must be handled carefully at this temperature to minimize bending. Within the SPF range, the SPF metals have the consistency of taffy, so bending can easily occur unless the operators take suitable precautions. Bending generally ruins the part because the part assumes the wrong aerodynamic shape or has unintended areas of stress concentration.
U.S. Pat. Nos. 4,622,445 and 5,683,608 describe improvements for an SPF process coupling the use of ceramic dies with induction heating. With an inductively heated SPF press or workcell, the sheet metal workpiece (or a susceptor surrounding the workpiece) is preferentially heated using an oscillating magnetic field without heating the platens or dies significantly. The ceramic dies are an insulator and retain heat induced in the part. Heating is easily controlled by stopping the induction. The part can cool relatively quickly even before removing it from the die. In Boeing's induction heating workcell, less energy is wasted because we do not heat significantly the large thermal mass of the platens and dies. Press operators need not work around hot dies and platens. Boeing also saves time and energy when changing dies to set up manufacture of different parts. The dies and platens are significantly cooler than those in a conventional SPF press, so they can be handled sooner, reducing the die change by several hours. Therefore, the induction heating process is an agile work tool for rapid prototyping or low rate production with improved efficiency and versatility. We also incorporate these patents by reference.
U.S. Pat. Nos. 3,920,175 and 3,927,817 describe typical combined cycles for SPF forming and diffusion bonding. Diffusion bonding is a notoriously difficult and temperamental process, especially for aluminum, that has forced many SPF fabricators away from multisheet manufacturing or to “clean room” production facilities and other processing tricks to eliminate the possibility of oxidation corrupting the bond. In addition, diffusion bonds are plagued with microvoids, which are difficult to detect nondestructively, but, if present, significantly diminish the structural performance of the joint. Even when it works, diffusion bonding is a time consuming process. The part typically must be held at elevated temperature and elevated pressure (about 400 psi) for several hours. For example, in U.S. Pat. No. 3,920,175, the diffusion bonding operation takes five hours at 1650° F. (900° C.), making the complete cycle forming and bonding each part six hours. In U.S. Pat. No. 3,927,817, diffusion bonding occurs prior to forming, but still requires four to five hours, resulting in a six hour bonding/forming cycle where the temperature must be held at 1650° F. (900° C.) for the entire period. Typically a hot press diffusion bonding process for common titanium alloys used in aerospace applications will require eight hours or more at 2500 psi and 800° C. (1472° F.), about six hours at 400 psi and 900° C. (1650° F.), or about two hours at 250–300 psi and 950° C. (1742° F.). Producing this heat and pressure for this length of time is expensive. The equipment and facilities to house it are expensive. The consumption of resources is large. The process limits the rate of production and is far from lean or agile.
Another diffusion bonding process uses a CRES template to apply pressure in the desired locations in the multisheet part is described in U.S. Pat. No. 6,129,261. Titanium alloys especially are amenable to this improved SPF/DB process because they can be diffusion-bonded at relatively low contact pressures. Aluminum alloys have a stable Al2O3 surface film and low oxygen solubility, and require relatively high pressures for diffusion bonding. The template reduces the processing cycle by focusing pressure on the areas where diffusion bonds are intended.
U.S. Pat. No. 5,420,400 describes a timesaving process for combining SPF with brazing, an operation that promises higher quality parts at lower production costs than diffusion bonding, because there is higher confidence in the integrity of the brazed joint than a diffusion bond. The SPF-brazing process also provides significant energy savings and shorter production times. The induction heating press or workcell can rapidly change the temperature of the part on which it operates. The troubles of diffusion bonding are eliminated by replacing diffusion bonding with brazing so that a much more efficient manufacturing cycle is possible. Manufacturers have greater assurance in the integrity of the brazed bond, can achieve a satisfactory brazed bond quickly and reliably, and can process the multisheet pack with a single heating cycle without removing the pack from the press. Conventional processing requires a significantly higher investment in capital equipment and usually requires the use of separate equipment maintained at the different temperatures to produce parts that require multiple, elevated temperature manufacturing operations. Combined heating cycles, like that used for the SPF-brazing process, reduce hand labor, capital equipment cost, and energy consumption.
A combined process for superplastic forming (SPF) and brazing preferably begins by assembling a pack of SPF sheet metal sheets having braze alloy placed where braze joints will be located in the finished part. The pack is inductively heated to the superplastic forming range, and formed to define the braze joints. After forming, the temperature is increased to reach the brazing temperature or melting point of the braze alloy to allow the alloy to flow in the area of the braze joint. Cooling the part below the superplastic forming range sets the braze joint and completes the process.
Manufacture of SPF/DB laser welded (LW) parts is described in U.S. Pat. No. 5,994,666. Weld cratering and tight radii at the start and stop of the weld are inherent limitations of laser welding. They result from the high intensity, narrowly focused beam, and have in the past resulted in sharp termination points that were areas of concentrated stress. The laser naturally produces a “keyhole” weldment that forms a crater at the weld termination, severely undercutting the top sheet at the end point of a stitch weld. Such welds weaken the top sheet of the core stack at the weld termination at a point that experiences high stress during inflation by gas pressure during superplastic forming. The SPF/DB/LW production process eliminates these weak points at the beginning and terminating ends of the weld by using a traveling laser welding head having a pressure foot for pressing the sheets into intimate contact around the region of the weld to ensure good weld quality. Usually stop-off is applied to the sheet interface to prevent later diffusion bonding, and the sheets are laser-welded through the stop-off.
If sealed openings through the sandwich structure are needed for fasteners, fluid or electric lines, control cables, or the like, a laser weld may be made in the full pack before it is superplastically expanded to seal weld around the region where a hole will be cut. The hole can then be cut inside the seal weld to produce a sealed opening through the full pack. A reinforcing tube having a length equal to the height of the die cavity (i.e., the thickness of the completed panel) is placed in the hole, and the pack forms around the tube as it inflates. The sheets usually will diffusion bend to the tube. The resulting panel has reinforced sealed openings of the desired diameter in the sandwich structure. Hard points can be made in a similar process by including a solid block at a predetermined location.